This invention relates to a pedestal crane and, more particularly to a crane designed for operations on offshore drilling and production platforms and other marine structures.
Conventionally, a number of exploration and completion operations are conducted with the use of independently air-powered and chain-operated hoists. These operations include blowout preventor handling, wellhead installation and maintenance, installation of topside components from jackup drilling units. Generally, two hoists are used in tandem; they perform a number of operations, for example raising and lowering of blowout preventors to and from the wellhead and transporting the blowout preventors to and from a storage location on the rig.
When the hoisting operations are conducted, other operations in the vicinity and on the drill floor are usually suspended, causing loss of productive rig time, creating a potential for hazardous conditions for the rig personnel, especially when heavy pieces of equipment are transported by overhead cranes.
Hoisting and handling of many components below the drill floor usually requires elaborate rigging with snatch blocks, slings and the like. Personnel on the rig floor and in the platform/cellar area usually communicate by radio or telephone, which adds to the complexity of performing a simple jacket topside operation.
In many cases, equipment delivered to an offshore platform is raised from the boat onto the deck with the deck cranes. The delivered equipment is then stored on deck, requiring additional maneuvering for installation in place. The double handling of the loads, particularly in jacket topside operations uses valuable rig time, increases the potential for damaging expensive equipment, creates potential hazard to the personnel and thus is not very efficient.
In view of the severe space restrictions, cranes for offshore facilities have to be compact. For example, U.S. Pat. No. 4,271,970 issued on Jun. 8, 1991 discloses a compact pedestal crane which is mounted on a drill rig floor. A dynamic load compensator absorbs sudden shock loads due to wave motions. However, despite the compactness of the design, this crane will still occupy valuable platform deck space.
U.S. Pat. No. 5,487,478 issued on Jan. 30, 1996 discloses a crane with an inverted kingpost. The crane has upper and lower bearings constructed of relatively soft bearing material that cooperates with relatively hard kingpost and upperworks structures. The design allows for access to conduct inspection or maintenance of components. Other U.S. patents, for example U.S. Pat. Nos. 3,898,847; 3,949,693; 4,576,518 and 4,589,801 all teach offshore oil rig structures with cranes.
The present invention contemplates elimination of drawbacks associated with the prior art and provision of a crane suitable for use on a jack-up drilling unit where it allows handling of loads from supply boats to the platform in an efficient manner, and other jacket topside load handling activities.
It is, therefore, an object of the present invention to provide a hoisting mechanism for an offshore jackup drilling unit.
It is another object of the present invention to provide an inverted pedestal crane that can be suspended from the underside of the rig floor to allow topside operations to be conducted contemporaneously with drill floor operations.
It is a further object of the present invention to provide an inverted pedestal crane that is hydraulically operated independently of a rig floor activity below the rig floor area.
These and other objects of the present invention are achieved through a provision of an inverted pedestal crane that has a base mountable to an underside of a horizontal support structure, such as for example a rig floor. A crane pedestal extends downwardly from the base, and a boom connects pivotally to the pedestal. The pedestal and the crane base rotate about a vertical axis, while the boom moves pivotally in relation to the pedestal. The crane has one main and a pair of auxiliary hydraulic rams for moving the boom during load handling operations.
The crane uses no counterweights. The overturning moments are efficiently handled by the supporting truss work that is secured to the pedestal and to an underside of the base, resisting overturning moments acting on the crane during load handling. The truss work, in turn, houses an hydraulically driven, rotating pinion drive mechanism, which is engaged to an oversize base. The base carries the inverted crane components. The oversize base and the truss work further enhance functionality of the crane by offsetting any radial point loadings and transmitting the moment to the slew bearing.
A pin and yoke of the main hydraulic ram cylinder are located in close proximity to the outside circumference of the base, thereby improving the operation of a robust slew bearing assembly that is mounted above the crane pedestal. The slew bearing assembly has a pair of parallel plates, a plurality of centering rollers and a plurality of stabilizing Hillman-type rollers mounted adjacent to an outer circumference of the parallel plates, between the plates.
A winch assembly is mounted on top of the base, the winch assembly carrying a cable that extends over a pulley secured to a free end of the boom. A truss assembly secured to the pedestal and to an underside of the base resists overturning moments acting on the crane when the boom engages a load. An operator cab is mounted on a support plate adjacent a lower portion of the pedestal.
The crane is adapted for operation under conditions where space is at a premium, such as for example an offshore platform. Of course, the crane can be used in other applications, its main advantage being its capability to be suspended from an underside of a horizontal surface, the top of which can be used for other operations and structures.